Hemodialysis is a process or procedure by which an artificial kidney replaces the function of a patient's kidney. Hemodialysis removes waste products such as creatinine and urea, by circulating blood outside the body through an external filter, called a dialyzer, that contains a semipermeable membrane. The blood flows in one direction and a dialysate flows in the opposite direction across the filter. The counter-current flow of the blood and dialysate maximizes the concentration gradient of solutes between the blood and dialysate, which helps to remove urea and creatinine from the blood.
Hemodialysis machines perform dialysis by pumping the patient's blood and dialysate through the dialyzer. In the use of such machines, blood is removed from the patient's vascular system via suitable equipment such as an arterial needle, supplied to the machine via a tubing, passed through the machine's dialyzer, and is returned to the patient via a venous needle for normal circulation through the patient's vascular system. Many dialysis patients have an arteriovenous shunt implanted to create a location having a high blood flow that simplifies the withdrawal of blood through a line connected to the part of the shunt that is closer to the arterial side of the shunt and the return of purified blood through a line connected to the shunt downstream of the withdrawal site, closer to venous side of the shunt.
While hemodialysis treatment achieves its desired beneficial effect various physiological parameters are affected during a treatment session which are relevant to cardiovascular stability of the patient undergoing the treatment. The dialysis treatment causes changes in body temperature and osmolarity. Potential problems caused by dialysis include acetate-induced vasodilatation, bio-incompatibility reactions, and malfunctions of the autonomous nervous system. It also has been found that symptomatic hypertension occurs in during a phase of about 20% of all dialysis treatments. This is because stress is placed on the blood pressure control system of the body due to the reduction of blood volume that is caused by ultra-filtration of the blood during the dialysis process. It is generally accepted that blood volume reduction during the dialysis process is an important, and may be even the most significant, factor for triggering the drop of the patient's blood pressure. Therefore, it is highly desirable, to be able to monitor the blood volume of a patient undergoing a dialysis treatment on a real-time basis so that changes in the treatment can be promptly undertaken to avoid and counteract any adverse affects to the patient.
It is generally known that the blood volume of a patient cannot be directly measured as an absolute quantity. Instead, techniques have been developed that give indications of the blood volume based on constituents contained in the blood. One such constituent is the level of hematocrit present in the blood. At page 10 of an article by Diana Hlebovy, “Hematocrit Based Blood Volume Monitoring and the Hemodialysis Patient”, 2010, of Henna Metrics, LLC, 695 North, 900 West, Kaysville, Utah 84037-4118 a formula is given that is used to determine the percent blood volume change based on the hematocrit change. Accordingly, it would be desirable to be able to measure is the hematocrit level during a dialysis procedure to provide an indicator of the patient's blood volume. It also would be desirable to accomplish this on a continuous real time basis that is non-invasive, meaning that there is no contact of any part of the measuring apparatus with the blood or the dialysate. It is even more desirable that this be accomplished without the need for additional or changes in the deformable tubing in which the blood circulates during a dialysis treatment
The “sound velocity” of blood has been used to measure various constituents and properties of blood. The term “sound velocity” is a recognized shorthand expression describing a characteristic of the speed at which sound waves travel in a medium. The speed of sound varies depending on the medium through which the sound waves pass. Sound velocity often is a parameter used in describing properties of different substances or mediums. Knowing the value of the sound velocity of a particular medium permits different characteristics or properties of the medium to be determined by using various mathematical algorithms.
U.S. Pat. No. 5,453,576 describes an apparatus and method for measuring hemodynamic blood parameters that includes a sound velocity sensor coupled to at least one of an arterial or venous portion of a patient's blood system. The sensor measures the sound velocity of the blood. An indicator medium is injected into the blood system upstream of the sensor to dilute the bloodstream, and the sensor detects the dilution through changes in measured sound velocity. The sound velocity changes are recorded and used to determine blood hemodynamic parameters.
In U.S. Pat. No. 5,685,989 the measurement of blood flow in a dialysis shunt is obtained by injection of an indicator material into a venous line leading from dialysis equipment to a shunt in the arm of the patient. The blood flow in an arterial line leading from the shunt at a location downstream of the venous line to the dialysis machine is monitored by an arterial line sensor for the presence of the indicator material. A detector connected to a sound velocity sensor provides a dilution curve in response to the change of the sound velocity of the blood due to the presence of the indicator material. The blood flow in the shunt is calculated from the area under the dilution curve.
While such apparatus and methods provide useful information about a patient's blood, they are somewhat difficult to use, particularly on a continuous basis. Accordingly, it would be desirable to have a method and apparatus in which a constituent of blood can be monitored on a continuous basis that is simple to use and does not require the use of a diluent.